I had a situation in our 8 player game after we drew the Tainted rations Mythos card, that had significant consequences dependent on how we chose to implement the Mythos card effect in relation to a couple of the investigators. In case it crops up again I would like some indication if we played it right.

Tainted Rations

Each Investigator loses 3 health unless he gains a Poisoned condition

Zoey Samaras was down to 3 stamina and already had a Poisoned condition. Does she lose her last 3 stamina because she cannot gain a Poisoned condition due to already having it or does she avoid taking 3 stamina loss because she already satisfies the criteria of having a Poisoned condition.

This was important as her life was literally on the line. We chose to avoid inflicting 3 stamina on her as she was already poisoned. It seemed that the criteria was Be poisoned or suffer 3 stamina as opposed to the actual action of physically drawing a poisoned condition from the deck to bestow upon her.

Wendy AdamsShe chose to gain the poisoned condition and then using her passive ability chose not to gain it. In essence she chose to gain the condition so she could choose not to be affected by it. which seems bizarre. Is this an allowed action as part of the rules.

As there was only 4 poisoned condition cards in the deck this allowed another investigator on low stamina to take the poisoned condition that Wendy would have had. Hopefully we played it right.

From what I remember, in your case, Zoey should have taken the 3 damage, and Wendy could ignore it.

When an effect forces you to take some bad stuff to avoid some other bad stuff, if you cannot take the bad stuff, the other bad stuff is mandatory. If you have a way of preventing the bad stuff, then you're safe.

So, since Zoey cannot be poisoned again, she must take 3 damage — and, in this case, die a horrible death —, and since Wendy can prevent the poisoned condition, and is herself not poisoned, she can safely choose the first option.

Notice that it would be different if you had the option of taking some bad stuff to get something (like "get delayed to upgrade will" effects). In those cases, the bad stuff is a condition for the effect bound to it.

Q. Can Wendy Adams’s passive ability stop her from gaining aCondition she chooses to gain?

A. Yes. If an effect allows Wendy Adams to gain a Condition aspart of a cost, such as “you may gain a Madness Condition” or“unless you gain a Blight Condition,” she may use her passiveability to not gain that Condition instead. If she does, the costof gaining the Condition has still been paid.

The game is designed in such a way that falling below 4 Stamina or Sanity puts you in the "kill zone". Never allow your character to end a turn, or have an Encounter in such a situation unless you are prepared for the consequences.

With regard to your questions, you have it right and should you wish to check you will find the answers in the FAQ.

page 3 of the FAQ wrote:

Q. If an investigator chooses an effect, can that effect be prevented?A. Yes. The effect an investigator chooses can be prevented byanother effect. In such a case, the investigator’s choice is notnegated. For example, an investigator must discard his Spellunless he loses 2 Sanity. If that investigator chooses to lose 2Sanity, that loss of Sanity can be prevented with the WhiskeyAsset or the Instill Bravery Spell.

Wendy is a little bit tricky as there are multiple scenarios that can arise with her specifically.

1. "You may gain Condition A to X" (you do not have Condition A)Possible result: You choose to gain Condition A, the trigger your passive ability to not receive Condition A and trigger X

2. "You may gain Condition A to X" (you do have Condition A)Result: You cannot choose to gain a Condition you already have, not even Wendy can do that, you cannot trigger X

3. "Trigger X unless you gain Condition A" (you do not have Condition A)Possible result: You choose to gain Condition A, the trigger your passive ability to not receive Condition A and do not trigger X

4. "Trigger X unless you gain Condition A" (you do have Condition A)Result: You cannot choose to gain a Condition you already have, not even Wendy can do that, you must trigger X

and now the most interesting one:

5. "You may gain Condition A. If you gain the Condition, X..."If you triggered your passive ability you effectivaly did not gain the Condition and do not trigger X

There is no choice to eat the tainted rations or not eat them -- everyone ate them. What happens next depends on what kind of shape you were in. If you were already poisoned from some other source, you are not in good shape. It's going to hurt you to eat spoiled food.